Negative electrode for lithium secondary battery

ABSTRACT

The present invention provides a negative electrode for a lithium secondary battery and a lithium secondary battery having the negative electrode. The negative electrode includes an aluminum alloy powder as an active material,  
     wherein the alloy is substantially amorphous, and is represented by the formula Al x Si y M z , where M is at least one transition metal selected from the group consisting of Ni, Co, Cu, Fe, Cr and Mn;  
     x, y and z are 40≦x≦80; 10≦y≦50 and 1≦z≦20, respectively, and x+y+z=100; and  
     average particle diameter of the alloy is not greater than 50 μm.

FIELD OF THE INVENTION

[0001] The present invention relates to a negative electrode for alithium secondary battery that includes an aluminum alloy as a negativeelectrode active material. More specifically, the present inventionrelates to an improved negative electrode active material which providesa negative electrode with which it is possible to prepare a lithiumsecondary battery having excellent charge-discharge cyclecharacteristics.

BACKGROUND OF THE INVENTION

[0002] When a lithium metal plate is used for a negative electrode for alithium secondary battery, active dendritic lithium is deposited outduring charge, and the deposited lithium reacts with the electrolyte todecrease the negative electrode capacity and causes an internalshort-circuit of the battery when it grows by repeated charging anddischarging. If a lithium-aluminum alloy sheet that is electrochemicallyprepared from lithium and crystalline aluminum is used instead of alithium metal sheet, reaction of lithium with an electrolyte andformation and growth of dendritic lithium by repeated charge-dischargecycles are inhibited, and charge-discharge cycle characteristics areimproved. However, electrochemical reaction (alloying reaction) rate oflithium and crystalline aluminum is low, and a dramatic improvement ofcharge-discharge cycle characteristics cannot be expected.

[0003] It has been proposed that a lithium-aluminum alloy sheet that isprepared electrochemically from lithium and amorphous aluminum be usedfor a negative electrode for a lithium secondary battery instead of theabove-explained lithium aluminum alloy sheet (Japanese Patent Laid-openPublication No. 63-13267). According to this publication, the reactionrate of the electrochemical reaction between lithium and amorphousaluminum is higher than that between lithium and crystalline aluminum,and charge-discharge cycle characteristics are improved as well.

[0004] However, the inventors of the present invention found thatinactive Li₂O is deposited out on the surface of a negative electrodebecause the lithium aluminum alloy sheet has a small contact (reaction)area between the alloy sheet and an electrolyte. Therefore, the obtainedcharge-discharge cycle characteristics are not satisfactory.

OBJECT OF THE INVENTION

[0005] Objects of the present invention are to provide a negativeelectrode for a lithium secondary battery that has excellentcharge-discharge cycle characteristics and to provide a lithiumsecondary battery which includes the negative electrode.

SUMMARY OF THE INVENTION

[0006] The present invention provides a negative electrode for a lithiumsecondary battery comprising aluminum alloy powder as an activematerial,

[0007] wherein the alloy is substantially amorphous and is representedby the formula Al_(x)Si_(y)M_(z);

[0008] where M is at least one transition metal selected from the groupconsisting of Ni, Co, Cu, Fe, Cr and Mn;

[0009] x, y and z are 40≦x≦80; 10≦y≦50 and 1≦z≦20, respectively, andx+y+z=100; and

[0010] average particle diameter of the alloy is not greater than 50 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a cross-section of a lithium secondary battery preparedin the examples.

[0012] The following elements are shown in the drawings:

[0013] A: lithium secondary battery

[0014]1: a positive electrode

[0015]2: a negative electrode

[0016]3: a separator

[0017]4: a positive electrode can

[0018]5: a negative electrode can

[0019]6: a positive electrode current collector

[0020]7: a negative electrode current collector

[0021]8: insulation packing

DETAILED EXPLANATION OF THE INVENTION

[0022] A substantially amorphous aluminum alloy is used in the presentinvention because the amorphous alloy has good ductility and does noteasily form a fine powder as compared to crystalline aluminum alloy. Acrystalline aluminum alloy tends to form a fine powder by volume changeswhen lithium is inserted and released during charge-discharge cycles.Powdering increases contact resistance between alloy particles anddecreases electron conductivity, and charge-discharge cyclecharacteristics are easily deteriorated. In the present invention, asubstantially amorphous aluminum alloy means that a powder X-raydiffraction pattern shows halo portion and an amorphous degree (A) asdefined by the following formula is equal to or greater than 0.3. Agreater amorphous degree (A) means more amorphousness.

[0023] Amorphous degree (A)=strength of highest peak of profile in haloportion/strength of highest peaks of all profiles

[0024] The substantially amorphous aluminum alloy can be prepared by aliquid quenching method, a vacuum distillation, an ion plating method, amechanical alloying method, and the like. The liquid quenching method ispreferable because it is low in cost and can be used in mass production.The liquid quenching method is a rapid solidification method, forexample, a roll method (an alloy is melted by heat to prepare a melt,the melt is transferred onto a rapidly spinning copper roller (singleroll method or double roll method)), a gas atomization method (the meltis atomized with an inert gas), and the like.

[0025] Powder aluminum alloy is used because the contact (reaction) areais greater than an aluminum alloy sheet and charge-discharge efficiencyis better than with a sheet. When an aluminum alloy sheet is used,electrically inactive Li₂O is deposited on the negative electrode tocause a decline in charge-discharge efficiency because the contact(reaction) area between the alloy and an electrolyte is small and thusthe electric current density is large. The average particle diameter ofthe powder aluminum alloy is preferably not greater than 50 μm. If theaverage particle diameter is greater than 50 μm, the powder easilybecomes fine. A smaller average particle diameter is more preferable.However, it is normally difficult to obtain a powder with an averageparticle diameter smaller than 3 μm because the aluminum alloy has alarge ductility.

[0026] The substantially amorphous aluminum alloy in the presentinvention is represented by the formula Al_(x)Si_(y)M_(z);

[0027] where M is at least one transition metal selected from the groupconsisting of Ni, Co, Cu, Fe, Cr and Mn; and

[0028] x, y and z are 40≦x≦80; 10≦y≦50 and 1≦z≦20, respectively, andx+y+z=100.

[0029] Si makes the specific capacity of the alloy smaller. When y isless than 10, it is difficult to increase sufficiently the specificcapacity. When y is greater than 50, an amorphous degree (A) of thealloy is small. Therefore, when y is out of the range, charge-dischargecycle characteristics decline. The transition metal M makes theamorphous degree (A) of the alloy greater. When z is less than 1 orgreater than 20, it is difficult to obtain a substantially amorphousaluminum alloy, and charge-discharge cycle characteristics decline. Amore preferred range is 5≦z≦10.

[0030] To prepare a lithium secondary battery having excellentcharge-discharge cycle characteristics, it is also important to use apositive electrode active material having large electrochemicalreversibility for the positive electrode as well as the negativeelectrode active material of the present invention for the negativeelectrode. As a positive electrode active material, lithium cobaltate(LiCoO₂), lithium nickelate (LiNiO₂), lithium manganate (LiMnO₂) and thelike, alone or in admixture, can be exemplified.

[0031] The electrolyte used in the lithium secondary battery of thepresent invention is not particularly limited. As a lithium salt for asolute of the electrolyte, there can be mentioned LiClO₄, LiCF₃SO₃,LiPF₆, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiBF₄, LiSbF₆, LiAsF₆, and the like.LiPF₆ or an imide represented by the formula LiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (1≦m≦4 and 1≦n≦4) is preferable to obtain a lithiumsecondary battery having a large discharge capacity.

[0032] As a solvent of the electrolyte, a cyclic carbonate, for exampleethylene carbonate, propylene carbonate, vinylene carbonate, butylenecarbonate and the like, and mixtures of the cyclic carbonate and alow-boiling point solvent, for example dimethyl carbonate, diethylcarbonate, methyl ethyl carbonate, 1,2-dimethoxyethane,ethoxymethoxyethane and the like can be illustrated.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0033] The present invention is described below in detail in conjunctionwith certain examples. However, it is of course understood that thepresent invention is not limited to the following examples. The presentinvention can be modified within the scope and spirit of the appendedclaims.

[0034] [Experiment 1]

[0035] A negative electrode of the present invention and a comparativenegative electrode and secondary batteries including them were preparedto compare charge-discharge cycle characteristics.

EXAMPLE 1

[0036] [Preparation of Positive Electrode]

[0037] 80 parts by weight of LiCoO₂ having an average particle diameterof 20 μm, 10 parts by weight of acetylene black (as a conductive agent)and 10 parts by weight of polytetrafluoroethylene (as a binder) weremixed and the mixture was pressed and cut to form a disc with a diameterof 17 mm to prepare a positive electrode.

[0038] [Preparation of Negative Electrode]

[0039] Al, Si and a transition element M as shown in Table 1 (the purityof each is 99.9 weight %) were weighed in an atomic ratio of 65:25:10and mixed in a mortar. After being press formed, an ingot was preparedby an arc melting method. The ingot was melted, and was solidifiedrapidly by a single roll method to prepare pieces of alloy. The alloypieces were crushed by a pin mill under argon atmosphere to preparealloy powders. It was confirmed that each alloy powder having an atomicratio of Al, Si and M of 65:25:10 had the formula Al₆₅Si₂₅M₁₀ byemission spectroscopy (ICP). An average particle diameter of each alloypowder was 30 μm according to a particle distribution measurementequipment using a laser diffraction particle size analyzer.

[0040] 80 weight % of the alloy powder (as a negative electrode activematerial) and 20 weight % of polytetrafluoroethylene (as a binder) weremixed and pressed and cut to prepare a disc having a diameter of 17 mmto prepare a negative electrode of the present invention. The sameweight of the alloy powder was used for each negative electrode.

[0041] [Preparation of Electrolyte]

[0042] LiPF₆ was dissolved in a mixture of ethylene carbonate anddiethyl carbonate at a volume ratio of 1:1 in an amount of 1 mol/l toprepare an electrolyte.

[0043] [Preparation of Lithium Secondary Battery]

[0044] Coin shaped lithium secondary batteries A1˜A7 were prepared byusing the positive electrode, negative electrode and electrolyteprepared above. A fine porous film of polypropylene was used as aseparator.

[0045]FIG. 1 is a cross-section illustrating the prepared coin-shapedlithium secondary battery. The lithium secondary battery includes apositive electrode 1, a negative electrode 2, a separator 3, a positiveelectrode can 4, a negative electrode can 5, a positive electrodecurrent collector 6, a negative electrode current collector 7 and aninsulator packing 8 made from polypropylene. The positive electrode 1and the negative electrode 2 are housed in a battery can comprising apositive electrode can 4 and a negative electrode can 5 on oppositesides of the separator 3. The positive electrode 1 and the negativeelectrode 2 are connected to the positive electrode can 4 and thenegative electrode can 5 through the positive electrode currentcollector 6 and the negative electrode current collector 7,respectively, to provide a structure to the battery capable of beingcharged and discharged.

COMPARATIVE EXAMPLE

[0046] An aluminum disc having a thickness of 0.3 mm and a diameter of7.8 mm prepared by a liquid quenching method was inserted between twolithium discs each having a thickness of 0.1 mm and a diameter of 7.8 mmto prepare a disc of a negative electrode (a comparative negativeelectrode) having a thickness of 0.5 mm and a diameter of 7.8 mm. Thecomparative negative electrode capacity is the same as that of thenegative electrode of the lithium secondary battery A1 of the presentinvention.

[0047] A mixture of polytetrafluoroethylene (PTFE) and titaniumdisulfide (TiS₂) at a weight ratio of 0.1:99.9 was coated on one side ofa nickel wire mesh and pressed and cut into a disc of a diameter of 7.0mm to prepare a positive electrode. A lithium secondary battery S wasprepared in the same manner as lithium secondary batteries A1˜A7 exceptthat the positive and negative electrodes prepared above were used.

[0048] [Charge-discharge Cycle Characteristics]

[0049] A charge-discharge cycle test was conducted in which lithiumsecondary batteries A1˜A7 were charged to 4.1 V at 100 μA at 25° C., andthen were discharged to 2.8 V (this is a cycle). The number of cyclesrequired until the discharge capacity was reduced to 80% of the originaldischarge capacity (the first cycle) was determined.

[0050] A charge-discharge cycle test was conducted with the comparativebattery S in the same manner as the batteries A1˜A7.

[0051] The results are shown in Table 1. An amorphous degree (A) of eachnegative electrode active material (aluminum alloy or aluminum) used foreach battery is also shown in Table 1. The fourth column, “cycles”,shows relative indexes when the number of cycles of lithium secondarybattery A1 is taken as 100. TABLE 1 Amorphous Battery M in Al₆₅Si₂₅M₁₀degree (A) Cycles A 1 Cr 0.42 100 A 2 Ni 0.40 98 A 3 Co 0.41 98 A 4 Cu0.36 95 A 5 Fe 0.37 97 A 6 Mn 0.39 98 A 7 Atomic ratio of 0.42 99 Cr:Ni= 1:1 S — 0.22 42

[0052] As shown in Table 1, lithium secondary batteries A1˜A7 having anegative electrode of the present invention in which an amorphousaluminum alloy powder having an average particle size of not greaterthan 50 μm is used as a negative electrode active material have bettercharge-discharge cycle characteristics as compared to lithium secondarybattery S having a comparative negative electrode. It is believed thatthe reason why the lithium secondary battery S does not have goodcharge-discharge cycle characteristics is that the aluminum alloy of thenegative electrode has a small contact area (reaction area) andelectrochemically inactive Li₂O was deposited out on the negativeelectrode to dramatically reduce charge-discharge efficiency. Otherpossible reasons for the decreased charge-discharge cyclecharacteristics of lithium secondary battery S are considered to be thatthe amorphous degree (A) is small because aluminum used for the negativeelectrode was prepared by liquid quenching method without addition of arare earth element or transition metal element to the aluminum, andtitanium disulfide used as the positive electrode active material doesnot have good electrochemical reversibility.

[0053] [Experiment 2]

[0054] The relationship between the average particle size of thealuminum alloy and charge-discharge cycle characteristics was studied.

[0055] Lithium secondary batteries B1˜B4 were prepared in the samemanner as lithium secondary battery A1 except that the average particlediameter of the substantially amorphous alloy powder represented byAl₆₅Si₂₅Cr₁₀ was 3 μm, 15 μm, 50 μm or 60 μm, respectively. That is,only the average particle diameter of the alloy is different from thatof battery A1.

[0056] The charge-discharge cycle characteristics of the batteries wereevaluated in the same manner as Experiment 1 to obtain the number ofcycles until the discharge capacity was reduced to 80% of that in thefirst cycle. The results are shown in Table 2. An amorphous degree (A)of the negative electrode active material (aluminum alloy or aluminum)used for each battery is also shown in Table 2. The results for lithiumsecondary battery A1 are also shown. The fourth column, “cycles”, showsrelative indexes when the number of cycles of lithium secondary batteryA1 is taken as 100. TABLE 2 Average Particle Diameter of Aluminum PowderAmorphous Battery (μm) degree (A) Cycles B 1 3 0.42 100 B 2 15 0.42 100A 1 30 0.42 100 B 3 50 0.42 94 B 4 60 0.42 62

[0057] As shown in Table 2, lithium secondary batteries A1 and B1˜B3have excellent charge-discharge cycle characteristics as compared tolithium secondary battery B4. It is believed the reason whycharge-discharge cycle characteristics of lithium secondary battery B4were not good is that the alloy powder represented by Al₆₅Si₂₅Cr₁₀having an average particle diameter of 60 μm used as a negativeelectrode active material for lithium secondary battery B4 was powdered.Therefore, it is important to use an aluminum alloy having an averageparticle diameter of not greater than 50 μm.

[0058] [Experiment 3]

[0059] The relationship between x, y and z in formula Al_(x)Si_(y)M_(z)and charge-discharge cycle characteristics was studied.

[0060] Lithium secondary batteries C1˜C34 were prepared in the samemanner as lithium secondary battery A1 except that different alloypowders having an average particle diameter of 30 μm were used as shownin Table 3. Lithium secondary batteries C2˜C5, C14˜C17, C20˜C23, C25˜C28are batteries having negative electrodes of the present invention, andthe other batteries have comparative negative electrodes.

[0061] The charge-discharge cycle characteristics of the batteries wereevaluated in the same manner as Experiment 1 to obtain the number ofcycles until discharge capacity was reduced to 80% of that in the firstcycle. The results are shown in Table 3. An amorphous degree (A) of thenegative electrode active material (aluminum alloy ofAl_(x)Si_(y)Cr_(z)) used for each battery is also shown in Table 3. Theresults for lithium secondary battery A1 are also shown. The fourthcolumn, “cycles”, shows relative indexes when the number of cycles oflithium secondary battery A1 is taken as 100. TABLE 3 x, y and z inAl_(x)Si_(y)Cr_(z) Amorphous Battery x y z degree (A) Cycles A1 65 25 100.42 100 C1 30 60 10 0.26 67 C2 40 50 10 0.36 94 C3 50 40 10 0.38 98 C460 30 10 0.42 104 C5 80 10 10 0.32 82 C6 85 5 10 0.24 66 C7 40 59.5 0.50.13 52 C8 50 49.5 0.5 0.14 55 C9 60 39.5 0.5 0.17 58 C10 70 29.5 0.50.18 61 C11 80 19.5 0.5 0.20 63 C12 85 14.5 0.5 0.21 64 C13 40 59.0 1.00.23 63 C14 50 49.0 1.0 0.31 70 C15 60 39.0 1.0 0.32 73 C16 70 29.0 1.00.34 78 C17 80 19.0 1.0 0.35 80 C18 85 14.0 1.0 0.28 67 C19 40 55.0 5.00.26 66 C20 50 45.0 5.0 0.34 88 C21 60 35.0 5.0 0.38 100 C22 70 25.0 5.00.39 101 C23 80 15.0 5.0 0.42 104 C24 85 10.0 5.0 0.28 68 C25 40 40 200.31 72 C26 50 30 20 0.32 73 C27 60 20 20 0.33 77 C28 70 10 20 0.32 72C29 75 5 20 0.25 64 C30 40 38 22 0.12 52 C31 50 28 22 0.14 54 C32 60 1822 0.17 59 C33 70 8 22 0.19 61 C34 75 3 22 0.21 62

[0062] As shown in Table 3, lithium secondary batteries C2˜C5, C14˜C17,C20˜C23 and C25˜C28 have excellent charge-discharge cyclecharacteristics as compared to lithium secondary batteries havingcomparative negative electrodes. Therefore, it is important for thealuminum alloy to be used for a negative electrode that can provideexcellent charge-discharge cycle characteristics, to have x, y and z inthe formula Al_(x)Si_(y)M_(z) in a range of 40˜80, 10˜50 and 1˜20,respectively.

[0063] It was confirmed that when M is each of the other transitionelements, it is also important that x, y and Z of the aluminum alloy bein the above ranges.

[0064] [Experiment 4]

[0065] A positive electrode active material was studied.

[0066] Lithium secondary batteries D1˜D4 were prepared in the samemanner as lithium secondary battery A1 except that each battery has adifferent positive electrode active material, i.e., LiNiO₂, LiMnO₂, a1:1 weight ratio mixture of LiCoO₂ and LiNiO₂, and TiS₂, respectively,were used as shown in Table 3. An amount of the positive electrodeactive material was adjusted such that the positive electrode of eachbattery has the same original capacity.

[0067] The charge-discharge cycle characteristics of the batteries wereevaluated in the same manner as Experiment 1 to obtain the number ofcycles until discharge capacity was reduced to 80% of that in the firstcycle. The results are shown in Table 4. The results for lithiumsecondary battery A1 are also shown. The third column, “cycles”, showsrelative indexes when the number of cycles of lithium secondary batteryA1 is taken as 100. TABLE 4 Positive Electrode Battery Active MaterialCycles A 1 LiCoO₂ 100 D 1 LiNiO₂ 99 D 2 LiMnO₂ 99 D 3 Mixture of LiCoO₂& 99 LiNiO₂ (1:1 weight ratio) D4 TiS₂ 56

[0068] As shown in Table 4, lithium secondary batteries A1 and D1˜D3have excellent charge-discharge cycle characteristics as compared tolithium secondary battery D4. This is because TiS₂ used as the positiveelectrode active material for lithium secondary battery D4 does not havegood reversibility during charge-discharge cycles. Therefore, at leastone lithium-transition metal complex oxide selected from lithiumcobaltate, lithium nickelate and lithium manganate is also preferred forbest results.

ADVANTAGES OF THE INVENTION

[0069] The present invention can provide a negative electrode and alithium secondary battery having excellent charge-dischargecharacteristics.

What is claimed is:
 1. A negative electrode for a lithium secondarybattery comprising an aluminum alloy powder as an active materialwherein said alloy is substantially amorphous and is represented by aformula Al_(x)Si_(y)M_(z), where M is at least one transition metalselected from the group consisting of Ni, Co, Cu, Fe, Cr and Mn; x, yand z are 40≦x≦80, 10≦y≦50 and 1≦z≦20, respectively, and x+y+z=100; andaverage particle diameter of said alloy is not greater than 50 μm. 2.The negative electrode for a lithium secondary battery according toclaim 1, wherein said aluminum alloy is prepared by a liquid quenchingmethod.
 3. A lithium secondary battery comprising a positive electrode,a negative electrode and a non-aqueous electrolyte wherein an activematerial of said positive electrode comprises at least alithium-transition metal complex oxide selected from the groupconsisting of lithium cobaltate, lithium nickelate and lithiummanganate; and an active material of said negative electrode comprisesaluminum alloy powder; wherein said alloy is substantially amorphous andis represented by a formula Al_(x)Si_(y)M_(z), where M is at least onetransition metal selected from the group consisting of Ni, Co, Cu, Fe,Cr and Mn; x, y and z are 40≦x≦80, 10≦y≦50 and 1≦z≦20, respectively, andx+y+z=100; and average particle diameter of said alloy is not greaterthan 50 μm.
 4. The negative electrode for a lithium secondary batteryaccording to claim 3, wherein said aluminum alloy is prepared by aliquid quenching method.
 5. The negative electrode for a lithiumsecondary battery according to claim 1, wherein M is Cr.
 6. The negativeelectrode for a lithium secondary battery according to claim 3, whereinM is Cr.
 7. The negative electrode for a lithium secondary batteryaccording to claim 1, wherein 5≦z≦10.
 8. The negative electrode for alithium secondary battery according to claim 3, wherein 5≦z≦10.